1. Field of the Invention
This invention relates generally to an optical analog-to-digital converter and, more particularly, to an optical analog-to-digital converter that uses an upwardfolding, successive approximation conversion approach.
2. Discussion of the Related Art
Rapid advances in technology, particularly the need for greater processing speeds, increased channel bandwidths and improved transmission reliability, have resulted in a steadily growing focus on the optical domain and the vast potential therein. Optical technology, a relatively new field compared to electrical, radio frequency (RF)-based technology, lacks maturity in various areas. In particular, the promise of high-speed, large-bandwidth processing advances with digital optics has been hampered by the lack of many basic technologies readily available in the RF domain.
The optical analog-to-digital converter is one such area which has presented problems. In particular, conventional optical systems rely on digitization in the electrical/RF domain, resulting in slower, lossier and noisier conversions back and forth between the optical and RF domains. Moreover, conventional successive approximation analog-to-digital converters utilize a "downward-folding" approach to test the sampled analog signal against a specific threshold. If the sample is below the threshold, the bit is set to "low" and the sample is passed directly to the next stage. If the signal amplitude is above the threshold, the bit is set to "high" and the system reduces the sample amplitude by the threshold value before passing it on to the next stage. Each successive stage tests the sample against thresholds growing closer and closer to zero. The algorithm for such a converter is difficult to implement in the optical regime because simple, robust methods of subtracting optical signals from one another have not been adequately developed.
What is needed is an optical device which will convert an optical analog signal to an optical digital signal with little or no reliance on RF technology or subtraction techniques, will eliminate system complexity and will allow for high speed, large bandwidth processing advances.